Entering a new era of green energy, several criteria such as cost, cycle life, safety, efficiency, energy, and power need to be considered in developing electrical energy storage systems for transportation, such as electric vehicles and grid storage. Li—S batteries are one of the prospective candidates in this regard as sulfur offers a high theoretical capacity of 1675 mAh g−1 at a safer operating voltage range of ˜2.1 V and lower cost compared to the currently used oxide and phosphate cathodes. With this perspective, there is increasing interest in recent years in Li—S battery research. Development of a high capacity (>800 mAh g−1) Li—S system with a long, acceptable cycle life will gives this system a greater opportunity to be commercialized in the near future.
The early-stage research in lithium-sulfur batteries was initiated three decades ago, but the spotlight did not return to this battery system until there was a renewed interest in electric vehicles (EVs) in recent years. The major impediments to the development of Li—S batteries are low active material utilization, poor cycle life, and low charge efficiency. The poor utilization of active material results from the insulating sulfur, which hinders the electron transfer during electrochemical reactions. Also, sulfur molecules form easily-dissolved polysulfide intermediates with lithium (Li2Sx, 2<x≦8) in the electrolyte, resulting in severe, irreversible capacity fade. The soluble polysulfides shuttling between the anode and cathode lead to low Coulombic efficiency. Thus, improving the conductivity of the sulfur cathode and maintaining/reutilizing soluble polysulfides within the cathode structure are critical to develop a viable Li—S system.
Many approaches have been explored to tackle the drawbacks of sulfur cathodes, such as synthesizing carbon-sulfur composites and applying surface coatings of conductive polymers. The studies have shown promising improvements in Li—S batteries, but the material processing steps are often elaborate and costly, limiting the feasibility of manufacturing a viable lithium-sulfur cell. Employing sulfur-carbon composites and applying conductive polymer surface modification are the main approaches in laboratories around the world to realize high capacity and improved cycle life. Both approaches enhance the electrical conductivity of the cathode and suppress the loss of soluble polysulfide intermediates during cycling and thereby improve the active material utilization and cyclability. In addition, the issue of low Coulombic efficiency has been resolved by the addition of lithium nitrate to the electrolyte. However, the major stream of Li—S battery research has focused on the modification “inside” of the cathode and electrolyte, but the design “outside” of the cathode such as cell configuration could be a new strategy for improving the performance of Li—S batteries.